Adiabatic frequency shifting in epsilon-near-zero materials: The role of group velocity

The conversion of a photon's frequency has long been a key application area of nonlinear optics. It has been discussed how a slow temporal variation of a material's refractive index can lead to the adiabatic frequency shift (AFS) of a pulse spectrum. Such a rigid spectral change has relevant technological implications, for example, in ultrafast signal processing. Here, we investigate the AFS process in epsilon-near-zero (ENZ) materials and show that the frequency shift can be achieved in a shorter length if operating in the vicinity of Ref"r g D 0.We also predict that, if the refractive index is induced by an intense optical pulse, the frequency shift is more efficient for a pump at the ENZwavelength.Remarkably, we showthat these effects are a consequence of the slowpropagation speed of pulses at theENZwavelength.Our theoretical predictions are validated by experiments obtained for the AFS of optical pulses incident upon aluminum zinc oxide thin films at ENZ.Our results indicate that transparent metal oxides operating near the ENZ point are good candidates for future frequency conversion schemes.